The present invention relates to chemical analysis and, more particularly, to devices for injecting samples into analytical equipment. A major objective of the present invention is to provide for improved manual injection of a sample into a gas chromatography (GC) system.
The contributions of the medical, environmental and life sciences to humanity have been facilitated by advances in chemical analysis. Many analytical techniques provide for the division of a complex sample into its components. Gas chromatography is one such analytical technique that separates volatized chemical components according to their relative partitioning between a gaseous mobile phase and a stationary (typically solid) phase. The solid phase is bound within a chromatography column, through which the mobile phase flows.
In gas chromatography, components in the mobile phase flow at generally the same rate. However, components favoring the stationary phase (having a lower partitioning constant) spend less time in the mobile phase, and thus have a lower average-flow (elution) rate through the column. Due to the different elution rates, sample components separate into bands, e.g., statistical distributions about a peak, as they progress through the column.
Preferably, separation continues until overlap between bands is negligible. This facilitates detection and quantification of sample components. In addition, components can be separately collected for further analysis. In general, greater separation can be achieved using narrower-bore columns, with capillary separation columns being state-of-the-art.
Especially with capillary columns, the volume of sample introduced is critical to effective component separation. Since the diameter of the column is a given, variations in sample volume are reflected in the initial "plug" length the sample occupies in the column. The shorter the initial plug, the more narrow will be the distributions about the peaks as the components separate. A short plug and, thus, a small sample volume is desired. However, if the sample volume is too small, some components can be difficult to detect; also quantification is less certain. Accordingly, the sample volume for capillary GC columns must be both small, e.g., less than 500 nanoliters, and precisely controlled.
Generally, a large volume can be injected with greater relative precision than can be a small volume. "Split-mode" injection systems divide a relatively large volume of sample between plural sample paths so that the volume in the one path through the column is a desired smaller volume. This is an expensive and complicated solution. In addition, the splitting process can introduce undesirable discrimination among sample components; this discrimination can occur where proportionally more of one component is vented than another component so that the composition of the sample in the GC column differs from its composition in the syringe. Accordingly, it is desired to provide for smaller sample volumes upon injection to avoid split-mode injections or to allow smaller split ratios to reduce component discrimination.
Control of sample injection volume pertains, in part, to syringe design. Syringes with capacities of 5 microliters or 10 microliters are common. These syringes are typically graduated to provide for measurement of smaller volumes. However, measurement of the small volumes is subject to parallax errors, which can affect the precision of sample volume determinations.
The manner in which the sample is injected also bears on the volume of sample injected. If the sample is ejected too slowly from the syringe needle so that it "drools" out, some of the sample can wick to the outside surface of the needle and/or to an injection port septum. The wicked volume might be lost, e.g., if it exits the port with the needle instead of through the column. Even worse, the wicked volume might flow through the column after a delay relative to the main body of the sample; this confuses detections and quantifications.
This drooling effect can be minimized by ejecting the sample at a sufficiently high velocity that it forms a jet as it leaves the needle. This "spitting" action can still leave some sample on the needle. However, selecting an appropriately high velocity achieves a "clean spit" so that sample does not remain on the needle.
On the other hand, as it was discovered in the course of the present invention, the ejection velocity can be too high in which case "overspitting" occurs. Since a syringe plunger does not extend into the needle, it is normal for some volume of sample to remain in the needle after injection. If the injection velocity is excessive, a portion of this sample volume that is supposed to remain is actually injected. Accordingly, injection velocity should be sufficient to attain a clean spit but not risk overspitting.
Furthermore, the delay between the needle's insertion into a GC port and injection of the sample must be kept small. Prior to insertion, a syringe needle is typically at room temperature, while the interior of the injection port is heated sufficiently to volatilize the sample. The needle begins heating as soon as it is inserted through the injection septum. If sufficient heating occurs prior to injection, some sample in the needle can volatilize and enter the column ahead of the bulk of the sample. The leading sample volume can then confuse detection and quantification.
Autoinjectors would appear to meet most of the criteria for sample injections into a capillary GC system. They can inject a sample soon after a needle penetrates an injection septum, and they can inject with high velocities in a repeatable manner. Nonetheless, there remains a problem with repeatability when it comes to injection of very small sample volumes. However, any problems facing autoinjection are dwarfed by those facing manual sample injection.
An autoinjector may be unavailable for reasons of cost, portability, downtime, and unsuitability for a given task. In these circumstances, manual injection is an attractive and sometimes necessary alternative. However, human physical control tends to be rather gross and slow relative to the demands of capillary GC sample injection.
Consider the case where a syringe is held by hand and the plunger operated by a finger or a thumb. There are difficulties ensuring the syringe holds the proper amount of sample: there is a parallax problem in reading the graduations; and there is a manual problem of controlling the plunger with the precision to match the desired graduation. Then there is a problem with alignment with the septum. A misaligned insertion can result in a damaged needle or plunger.
Assuming an aligned insertion, there can be a delay between the time the needle penetrates the septum and the time the plunger is operated to eject the sample; in the meantime, the needle can have heated to the point that some sample is volatized and carried down the column ahead of the main sample plug.
The most difficult problem would seem to be achieving the precise sample injection velocity. There can be large variations between human operators; and even variations in the injections of an individual operator. This variability can lead to some samples being drooled in, others being spit in, but not cleanly, and others being overspit. The desired clean spit is difficult to achieve manually on a repeatable basis.
What is needed is a system that allows for precise manual sample injection in general and specifically for capillary GC systems. The injections should occur soon after needle insertion through an injection septum. A precise volume of sample should be injected at a precise "clean-spit" velocity to optimize separation effectiveness. Finally, some improvement in the handling of small sample volumes by autoinjectors is desired.